Decreased yields in IC device fabrication processes often result from defects caused by misalignment of subsurface layers or patterns or by particulate contaminants. FIGS. 1, 2A, and 2B show repetitive electronic circuits 10 of an IC device or work piece 12 that are commonly fabricated in rows or columns to include multiple iterations of redundant circuit elements 14, such as spare rows 16 and columns 18 of memory cells 20. With reference to FIGS. 1, 2A, and 2B, circuits 10 are designed to include between electrical contacts 24 laser severable conductive links 22 that can be removed to disconnect a defective memory cell 20, for example, and substitute a replacement redundant cell 26 in a memory device such as a DRAM, an SRAM, or an embedded memory. Similar techniques are also used to sever links 22 to repair CCD imaging devices or to program a logic product, gate arrays, or ASICs.
Links 22 in link structure 36 are about 0.3 micron (μm)-2 μm thick and are designed with conventional link widths 28 of about 0.4 μm-2.5 μm, link lengths 30 between adjacent electrical contacts 24, and element-to-element pitches (center-to-center spacings) 32 of about 2 μm-8 μm from adjacent circuit structures or elements 34. Although the most commonly used link materials have been polysilicon, polycide, and like compositions, memory manufacturers have more recently adopted a variety of more electrically conductive metallic link materials that may include, but are not limited to, aluminum, chromide, copper, gold, nickel, nickel chromide, titanium, tungsten, platinum, as well as other metals, metal alloys, metal nitrides such as titanium or tantalum nitride, metal silicides such as disilicide, tungsten silicide, or other metal-like materials.
Electronic circuits 10, circuit elements 14, or memory cells 20 are tested for defects, the locations of which may be mapped into a database or program. Traditional 1.047 μm or 1.064 μm infrared (IR) laser wavelengths have been employed for more than 20 years to explosively remove conductive links 22. Conventional memory link processing systems focus at a selected link 22 a single laser output pulse 37 having a pulse width of about 4 nanoseconds (ns) to 30 ns. FIGS. 2A and 2B show a laser spot 38 of spot size (area or diameter) 40 impinging a link structure 36 composed of a polysilicon or metal link 22 positioned above a silicon substrate 42 and between component layers of a passivation layer stack including an overlying passivation layer 44 (shown in FIG. 2A), which is typically 500 Å-10,000 Å (D) thick, and an underlying passivation layer 46. FIG. 2C shows two adjacent links 22 separated by an intermediate passivation layer 48. Each of links 22 has opposite side surfaces 52 separated by a distance that defines a nominal link width 28, which laser spot 38 encompasses to sever link 22. Silicon substrate 42 absorbs a relatively small proportional quantity of IR laser radiation, and conventional passivation layers 44, 46, and 48 such as silicon dioxide or silicon nitride are relatively transparent to IR laser radiation. The links 22 are typically processed “on-the-fly” such that the beam positioning system does not have to stop moving when a laser pulse is fired at a selected link 22, with each selected link 22 being processed by a single laser pulse. The on-the-fly process facilitates a very high link-processing throughput, such as processing several tens of thousands of links 22 per second.
FIG. 2D is a fragmentary cross-sectional side view of the link structure of FIG. 2B after removal of link 22 by the prior art laser pulse. To avoid damage to the substrate 42 while maintaining sufficient laser energy to process a metal or nonmetal link 22, Sun et al. in U.S. Pat. Nos. 5,265,114 and 5,473,624 describe using a single 9 ns to 25 ns laser pulse at a longer laser wavelength, such as 1.3 μm, to process memory links 22 on silicon wafers. At the 1.3 μm wavelength, the laser energy absorption contrast between the link material 22 and silicon substrate 42 is much larger than that at the traditional 1 μm laser wavelengths. The much wider laser processing window and better processing quality afforded by this technique has been used in the industry for about five years with great success.
However, ever-shrinking link dimensions and link-to-link pitch sizes demand a smaller laser beam spot size. A shorter laser wavelength is thus preferred for delivering a smaller laser beam spot size. A shorter laser wavelength than the 1 μm and 1.3 μm will also deliver a better coupling of the laser energy into the link target material to facilitate the process.
U.S. Pat. No. 6,057,180 of Sun et al. describes a method of using ultraviolet (UV) laser output to sever links with the benefit of a smaller beam spot size. However, removal of the link itself by such a UV laser pulse entails careful consideration of the underlying passivation structure and material to protect the underlying passivation and silicon wafer from damage by the UV laser pulse.
U.S. Pat. No. 6,025,256 of Swenson et al. describes methods of using ultraviolet (UV) laser output to expose or ablate an etch protection layer, such as a resist or photoresist, coated over a link that may also have an overlying passivation material, to permit link removal (and removal of the overlying passivation material) by different material removal mechanisms, such as by chemical etching. This process enables the use of an even smaller beam spot size. However, expose and etch removal techniques employ additional coating, developing, and/or etching steps, which typically entail sending the wafer back to the front end of the manufacturing process for one or more extra steps.
FIG. 3A is the typical temporal shape of a traditional laser pulse at wavelengths of 1 μm and 1.3 μm used in the link processing. To more effectively use the laser energy, Smart et al. in U.S. Pat. Nos. 6,281,471 and 6,340,806 propose using laser pulses of temporal shape shown in FIG. 3B with substantially square temporal power density distributions to process the links. According to Smart et al., the rise time of the laser pulse has to be shorter than 1 ns, the flatness of the squared wave top has to be better than 10%, and the fall time has to be sufficiently short. The stated advantage of using laser pulses with the temporal shape shown in FIG. 3B was that the sharp rise time of the laser pulse would deliver thermal shock to the overlying layer of oxides and thereby facilitate the link blowing process. In addition, the reflectivity of the laser energy by the link at the higher power density would be reduced with the fast rising, short duration pulse. If, however, breaking the overlying passivation layer sooner with the help of a thermal shock wave delivered to the layer by the sharp rise time of the laser pulse truly facilitates the process, processing link structures with no overlying passivation layer would not have been a technical challenge. Industry practice has proved otherwise.
Because of inevitable variations of the link structure, such as, for example, the thickness of the overlying passivation layer; the thickness, width, and side wall slope of the link itself; and the thickness of the underlying passivation layer, there is a need for some head room in the laser pulse energy used to process the links. Typically, the link material will be totally removed well before of the laser pulse ends. Preferably, for the typical laser pulse used, the link material is totally removed by time t1, as shown in FIG. 3A. Similarly, time t1 in FIG. 3B depicts the time when the typical link material is totally removed. Persons skilled in the art will realize that the laser pulse energy after time t1 for both cases imposes a great risk of damaging the silicon substrate because there would be no link material remaining to shield the substrate from exposure to the laser energy. The laser pulse energy after time t1 imposes great risk of damaging also the neighboring structure to the link. Unfortunately, for the traditional laser pulse, there is no control over the temporal shape of the laser pulse after time t1. For the substantially square temporal laser pulse, it is worse in that right after the time t1 the laser pulse will remain at its peak intensity for a while, causing even greater risk of damage to the substrate or neighboring structure.
What is needed, therefore, is a better way of controlling the temporal power profile of the laser pulse to facilitate better link process quality and yield.